Elemental redistribution in tropical soils: insights into REE, U, and Th mobility after extended phosphogypsum use

Phosphogypsum is a soil conditioner used to enhance lime effects in depth and also to decrease aluminum (Al) toxicity to plants. It is a by-product from phosphate mining and can be enriched in some contaminants, such as rare earth elements (REE), uranium (U), and thorium (Th). However, there is a lack of studies evaluating the long-term effects of this practice, particularly in soil depths. To address this gap, this study aimed to evaluate the remobilization of REE, U, and Th in a highly weathered soil (Oxisol) for 9 years. Four soil parcels received a single application of 28 tons ha^-1 in different years: 2005, 2008, 2010, and 2013, with soil sampling occurring in 2014. Soil samples were digested and analyzed using mass spectrometry. The REE input from phosphogypsum reached 1681 mg kg^-1, of which 95.89% corresponded to light rare earth elements (LREE). Although the reference area showed high background levels, temporal enrichment occurred in the treated areas, especially for LREE in the 0-40 cm layer, with cerium (Ce) being the most abundant element. Over nine years, REE concentrations decreased, but the distribution patterns of REE, U, and Th revealed to be dependent on chemical characteristics, natural background in soil and content in the phosphogypsum. The ratio ΣLREE/ΣHREE normalized by the reference area increased over the years, meaning a preferential depletion of HREE. Over time, phosphogypsum inputs led to enrichment of Th and scandium (Sc), whereas U and Y showed higher mobility and losses. These findings highlight the temporal and spatial behavior of potentially toxic elements introduced via phosphogypsum and their implications for soil and environmental quality.